Einstein once described his friend Michele Besso as “the best sounding board in Europe” for scientific ideas. They attended university together in Zurich; later they were colleagues at the patent office in Bern. When Besso died in the spring of 1955, Einstein — knowing that his own time was also running out — wrote a now-famous letter to Besso’s family. “Now he has departed this strange world a little ahead of me,” Einstein wrote of his friend’s passing. “That signifies nothing. For us believing physicists, the distinction between past, present and future is only a stubbornly persistent illusion.”

Many physicists have made peace with the idea of a block universe, arguing that the task of the physicist is to describe how the universe appears from the point of view of individual observers. To understand the distinction between past, present and future, you have to “plunge into this block universe and ask: ‘How is an observer perceiving time?’” said Andreas Albrecht, a physicist at the University of California, Davis, and one of the founders of the theory of cosmic inflation.

Others vehemently disagree, arguing that the task of physics is to explain not just how time appears to pass, but why. For them, the universe is not static. The passage of time is physical. “I’m sick and tired of this block universe,” said Avshalom Elitzur, a physicist and philosopher formerly of Bar-Ilan University. “I don’t think that next Thursday has the same footing as this Thursday. The future does not exist. It does not! Ontologically, it’s not there.”

This article was reprinted on TheAtlantic.com.